A critical consideration in most sheet handling machines is the ability to rapidly feed individual documents through the machine, one at a time. For example, reproducing machines such as copiers, printers, and the like must include sheet feeding devices that are capable of rapidly and reliably feeding the individual sheets of a receiver medium (e.g. paper) through complex travel paths within the machine. For such a sheet-feeding device (i.e. sheet feeder) to be successful, it must be capable of operating at high transport speeds with only a minimum of downtime due to misfeeds or multifeeds (hereinafter collectively referred to as “misfeeds”).
Sheet feeders are typically of two types; friction feeders and vacuum feeders. Friction feeders have been proven reliable for feeding individual sheets in many applications but, unfortunately, are susceptible to significant misfeeds when subjected to the harsher conditions normally found in higher speed copiers and printers. Vacuum feeders are more reliable for high-speed applications but require more precise control in reducing the number of misfeeds.
Some more recent sheet feeders have combined the positive aspects of both friction and vacuum sheet feeders in order to reduce misfeeds in high-speed machines; see U.S. Pat. No. 5,295,676, issued Mar. 22, 1994. In this feeder, a “bottom-feed” sheet feeder is disclosed wherein the bottommost sheet of a stack of individual sheets in a tray is engaged and is removed by one or more belts. At the same time, a vacuum is applied through the belts to acquire and maintain the sheet against the belts as the belts moves the sheet from the stack and feeds into the machine.
A similar approach is disclosed in U.S. Pat. No. 5,634,634, issued Jun. 3, 1997, wherein a “bottom-feed” sheet feeder is disclosed for handling sheets after a first pass through the duplex copier. This feeding mechanism includes a vacuum corrugated duplex tray, which receives and stacks the sheets after the first pass during which information has been copied onto one side of each sheet. The feeding mechanism then feeds the stacked sheets, one at a time, back off the tray for a second pass through the copier so that additional information can be copied onto the respective sheets. This combined approach has also been used in “top-feed” sheet feeders; see in U.S. Pat. No. 5,334,133, issued Sep. 4, 1994.
In sheet feeders, such as described in U.S. Pat. Nos. 5,634,634 and 5,334,133, cited above, one or more perforated belts move in a closed loop over a ported plate which, in turn, is in communication with a vacuum plenum. As will be understood in the art, when the holes in the belts align with the ports in the ported plate, the vacuum in the plenum acts through the aligned holes to attract and hold an individual sheet (i.e. the bottommost sheet of the stack, if the feeder is a “bottom feed” or the topmost sheet, if the feeder is a “top feed”, hereinafter referred to as “acquired sheet”) against the moving belts.
A surface within the respective tray or hopper, which holds the stack of individual sheets, is configured (i.e. corrugated) to aid in separating the acquired sheet from the stack. At the same time, a positive air stream is directed onto the front of the stack to further aid in separating the acquired sheet from the sheets remaining in the tray.
In feeders of this type, it is desirable to limit the air flow through the belts in order to control the “air bleed” through the sheets, themselves. This can be a real problem where the sheets are comprised of thin/porous materials or where the sheets have pre-punched holes or the like near the lead edges thereof. If the air “bleeds” through the acquired sheet (e.g. through the pre-punched holes therein), the forces generated thereby can, and often does, attract and hold a second sheet against the acquired sheet thereby resulting in a dreaded misfeed.
To alleviate this problem in the previous feeders, e.g. see U.S. Pat. No. 5,634,634, the ports in the vacuum plate are configured so that the flow of air into the plenum is restricted near the lead edge of the stack in the tray. That is, the ports adjacent the lead edge of the stack are made smaller than the other ports in the vacuum plate in order to minimize the air flow through the ports near the lead edge of the stack that may align with a hole or the like in the sheet and thereby reduce the attractive force on the sheets as they are acquired and moved by the belts.
Unfortunately, however, there still has to be vacuum area to tack the lead edge of the acquired sheet onto the belts. It has been found that, in order to do this, some of the smaller openings have to be enlarged (e.g. made triangular in shape) near the lead edge of the vacuum plate in order to insure that the holes in the belt near the lead edge are in communication with the vacuum plenum to provide the air flow (i.e. attractive force) needed to acquire and hold the lead edge of the acquired sheet against the belt during acquisition and removal.
While these known sheet feeders have been successful in most applications, problems with “air bleed” still exist, especially where the acquired sheet has pre-punched holes or the like near the lead edge thereof. While the smaller ports do limit the airflow near the lead edge, a real probability exists that, at some time, one or more of these ports (e.g. slightly larger, triangular ports) will directly align with a hole(s) in the belt and with a pre-punched hole(s) in the acquired sheet. If and when this occurs, the airflow through the belt can attract and acquire a “second” sheet in the stack thereby producing a misfeed. Further, the small ports used in this configuration are susceptible to becoming plugged with paper dust, etc. over long periods of operations which, in turn, can result in undesirable downtime of the machine.